In a conventional floating roof tank, with which the seal of the invention is particularly useful, there is provided a clearance space or rim space between the tank side wall and the vertical rim of the floating roof. It is necessary to provide clearance space to permit unrestrained vertical travel of the roof within the tank. The clearance space is of sufficient size that local dimensional variations in the circularity of the tank sidewall or shell, commonly called out-of-roundness, which can result from uneven foundation settlement, imprecise fabrication or erection or unusual live loads such as high winds and the like, do not hamper vertical travel of the roof.
The clearance space in essentially all floating roof tanks is covered in one way or another to prevent escape of vapor from the stored liquid through the clearance or rim space to the atmosphere. Various types of seals are used for this purpose. Many of the seals not only are intended to prevent vapor escape but also to center the roof so that it moves vertically relatively freely and prevents enlargement of the clearance space on one side of the roof since that could lead to escape of vapor by disengagement of the seal from contact with the tank wall.
Kinghorn et al U.S. Pat. No. 4,116,358 and Thiltgen et al U.S. Pat. No. 4,308,968 disclose floating roof clearance space seals made of metal sections. However, the seals include a gas impermeable fabric which extends continuously around the perimeter of the roof. The installation and maintenance of the fabric is expensive and time consuming but is essential to keep vapor from escaping between adjacent metal sections.
Kinghorn et al U.S. Pat. No. 4,191,303 also discloses a seal made of metal sections. However, the adjacent metal sections have overlapping edge portions with elongated holes. Fastening means, such as bolts, extend through the holes but permit the metal sections to slide laterally to accommodate dimensional change in the clearance or rim space. Even though a gasket is provided between the overlapping edge portions of the metal sections, to prevent vapor escape through the elongated holes and to the atmosphere large washers are needed to cover the elongated holes. Additionally, installation of the seal is time consuming because of the many bolts used, each of which requires precise tightening so as to permit lateral movement while keeping the gasket slightly compressed.
An additional problem with prior art seals is that the metal sections are made straight or flat and only become arced or bent when pressure is applied to them when installed or when the space between the roof edge and the tank wall is reduced during lateral movement of the roof during use. When the top edge of a metal section hits a protrusion, such as a weld or rivet, as the roof rises the metal section can buckle upwards and further lodge against the protrusion. Bending one or more of the metal sections in this manner is undesirable since it can damage the sections and open the seal.
Because of the described disadvantages and shortcomings in the previously available seals, a need exists for alternative seals for floating roof tanks.